Our atmosphere, as we noted yesterday, contains a myriad of tiny particles, many of which, depending on their size and distribution, have a profound effect upon our weather. Some of them, indeed, could be said to make the weather possible.
At one end of the scale are the specks of dust responsible for haze. These relatively large particles are most abundant in the first few hundred feet above the ground, and heavily contaminated air may contain anything from 10,000 to 50,000 of them in every cubic inch. They are, to quote Milton, "as thick and numberless as the gay motes that people the sunbeams", and they have many origins - from rocks and minerals in powdered form through specks of soot and particles of pollen to every sort of vegetable fibre.
But there are other, much tinier particles, that even when present in the very greatest numbers still remain invisible, suspended in an atmosphere that looks pristine pure and clean and clear. They are far more numerous than the specks of dust and some are liquid, or even gaseous, in form. They come from volcanoes, forest fires and ordinary chimneys; some are particles of sea salt and others are formed as a result of chemical reactions in the atmosphere involving substances like sulphur and nitrogen dioxide.
Many of these smaller particles act as "condensation nuclei" to play an important part in the development of clouds and fog.
Clouds form when the temperature of moist air falls to such an extent that it is no longer capable of holding the amount of water vapour it contained originally. At a certain temperature - known to meteorologists as the "dew point" - the moisture begins to condense into water droplets. But there is an important proviso to this process of condensation: it is essential for the moisture to have something tangible onto which it can condense.
Inside a house, window glass and cold walls provide the needful, and it is often on these surfaces that condensation first appears. In the atmosphere outside tiny nuclei perform the same function. The most favourable nuclei for fog and cloud formation are those that are "hygroscopic" - substances like salt and acid particles that attract and absorb the moisture from the air.
The particles themselves are so tiny as to be invisible to the naked eye - typically about a thousandth of an inch in diameter. So too are the minuscule droplets that form on them as a result of condensation, but the liquid spheres are big enough to scatter sunlight and make the resulting ensemble visible as a cloud.